The use of thermosettable resin compositions containing low molecular weight polyesters as protective coatings for substrates such as steel coil is known in the art. An ideal protective coating is one that adheres well to the substrate, is simultaneously hard and flexible, and resists solvents, abrasion, and dry heat. A combination of such properties in a polyester-based coating is ordinarily difficult to achieve because one property can often only be enhanced at the expense of another. For example, excellent coating flexibility is essential during substrate shaping, which normally follows coating application. High hardness is desirable for aesthetic reasons, and for greater durability and resistance to stains and solvents. Hardness may be enhanced by increasing the cyclic (i.e., the proportion of aromatic or cycloaliphatic recurring units) content of the polyester. However, a high cyclic content tends to result in inflexible or brittle coatings. Another way to increase hardness is to incorporate significant amounts of a polyol having three or more hydroxy groups into the polyester. The polyester is consequently highly branched rather than linear and tends to decrease the flexibility of the cured coating. Problems with premature gelation are sometimes also observed with such branched polyesters.
Ideally, polyesters are non-crystallizable and dissolve freely in common organic solvents. Resin compositions are normally applied in solution form in order to lower the viscosity of the neat polyester/aminoplast mixture. Since a polyester typically is stored for an extended period of time prior to application, it is important that the polyester not crystallize or precipitate out of solution. Increasing the cyclic content of a polyester to enhance the hardness of a cured coating, particularly if the dicarboxylic acid component is terephthalic acid, tends to increase the crystallizability of the polyester and to decrease its solubility in organic solvents.
The following representative coating patents illustrate the property trade-off dilemmas herein described.
U.S. Pat. No. 3,804,920 teaches the use of a blend of a high cyclic content polyester and a low cyclic content polyester in a resin coating composition.
U.S. Pat. No. 4,140,729 describes a resin composition containing a polyester with high cyclic content. Flexibility and non-crystallinity are provided by the incorporation of 1,6-hexanediol into the polyester.
U.S. Pat. Nos. 4,229,555 and 4,393,121 teach high cyclic content polyesters containing small amounts of an aliphatic dicarboxylic acid to enhance the flexibility of the cured resin composition.
U.S. Pat. No. 4,520,188 discloses a polyester comprised of neopentyl glycol, 1,4-cyclohexanedimethanol, aliphatic diacid, terephthalic acid, and at least one other aromatic dicarboxylic acid. This polyester gives coatings having a satisfactory overall balance of properties.
The above patents teach that it is desirable to have a high level of cycloaliphatic or aromatic diacid for linear polyesters because these acids contribute favorable hardness properties. Each of the above formulations suffers from the disadvantage of being complicated and/or derived from expensive components. The references overcome the crystallinity problem of linear, high-cyclic content polyesters either by using expensive flexibilizing diols such as 1,6-hexanediol and 1,4-cyclohexanedimethanol, or by including aliphatic diacids.
Japanese Kokai No. 59-91118 discloses polyester polyols containing 2-methyl-1,3-propanediol. The reference teaches that polyester polyols made using 2-methyl-1,3-propanediol are inexpensive and easily manufactured. The polyester polyols have low crystallinity and good solubility in curable compositions compared with polyesters derived from neopentyl glycol and the like. Branched polyesters derived from isophthalic acid, 2-methyl-1,3-propanediol, adipic acid, and either trimethylolpropane or pentaerythritol are illustrated. The polyesters of the examples have relatively low isophthalic acid contents of from about 28-36 mole percent. Linear polyesters diols derived from isophthalic acid are not shown, nor are polyesters having isophthalic acid contents of greater than about 40 mole percent.
Clearly, there is a need for linear polyester diol resin compositions that are easily prepared, solvent-stable, and non-crystallizable, especially at high isophthalic acid contents. There is also a need for coatings that are hard, non-brittle, glossy, and resist stains and abrasion.